This invention relates to the preparation of an alumina coated substrate and more particularly, to a method for improving the quality of the alumina coating on a substrate and to improve the efficiency of the coating process.
An alumina coated substrate which can be used for many applications including filtering gases and particles from the exhaust of an internal combustion engine or other source of gas is disclosed in coassigned U.S. Pat. No. 3,231,520. The alumina film is disclosed as being particularly useful as a support for a catalytic material which promotes various reactions, such as those useful in treating exhaust gases in order to clean-up or remove otherwise harmful pollutants.
Coassigned U.S. Pat. Nos. 3,227,659 and 3,410,651 disclose the impregnation of the calcined alumina film with phosphorus and chromium containing materials, respectively. British Pat. No. 1,271,710 discloses that the alumina coated substrate can be used by itself, without any added materials, as a filter to remove lead or lead compounds. The use of the alumina coated substrate to filter and remove carbon particles from diesel engine exhaust gases is disclosed in coassigned U.S. Pat. No. 4,039,294.
In the previously used methods, such as set forth in coassigned U.S. Pat. Nos. 3,231,520 and 3,240,698, a substrate was coated by contacting it with a sodium aluminate solution into which aluminum metal was also charged. The starting sodium aluminate solution was prepared by reacting aluminum metal with a relatively strong aqueous solution of sodium hydroxide or, alternatively, by dissolving alumina in strong aqueous sodium hydroxide. During the coating operation, the substrate along with a quantity of aluminum metal calculated to provide a coating of a desired thickness on the substrate, were both contacted with the sodium aluminate solution. After all of the aluminum metal reacted, the substrate was maintained in contact with the solution for a substantial period of time. This permitted as much of the aluminum hydrate as possible in the solution to precipitate out and coat onto the substrate. Such a process took several hours due to (a) the need to build up a highly supersaturated coating solution by reacting aluminum metal with the solution to initiate the coating, (b) the need to build up the coating solution concentration at a rate which did not overly supersaturate the solution and cause sudden precipitation of alumina hydrate from the solution, and (c) the need to maintain the substrate in the solution an additional length of time, after all of the aluminum metal has reacted, to let the concentration of the coating solution decrease to a level where the alumina hydrate stopped precipitating out and coating the substrate.